Doctor wont give a clear reason on HCG levels?

I have to go for my 3rd blood draw today. I was in the ER on Sunday for bleeding. They did a blood draw and ultrasound. U/s showed nothing because its too early to even detect something. And my HCG levels were at 116. The doctor in the ER said it looked like I was a week behind what I thought and to follow up with a OB. I had my appointment with the foctor yesterday and had a 2nd blood draw the day before. My levels only went up to 150. He said that that are still very low especially for this early. He has really said much on why they are so low and when I ssk what it could be, he tells me that it's still to early to tell anything. What are normal HCG levels for 4 1/2 weeks? Do I have a reason to be concerned? I feel like this pregnancy has already been so much stress and that i am going to have a complicated one.